The present invention relates to an apparatus for applying a chemical foam to a traveling substrate, and more particularly to such an apparatus that minimizes fluctuations in the application upon startup following a stoppage of the traveling substrate.
Chemicals are commonly applied to traveling substrates in the form of a foam medium to obtain uniform dispersion of a thin application of the chemical. For example, dyestuff, sizeing, and various treating chemicals are applied to textile fabrics of various sorts. To obtain uniform distribution across the traveling substrate and to obtain desired penetration, the foam is normally applied under pressure to the substrate by having the foam delivery slot of the applicator in sealed contact with the substrate.
Conventionally, the foam is fed from a generator through a delivery conduit to an applicator head at the location of application to the traveling substrate. The foam is a compressable medium, which results in a significant pressure drop between the generator and the delivery slot of the applicator head. In this type of system, during continuous application to a continuously traveling substrate, a uniform thickness of foam can be applied with the foam being delivered at a constant rate and pressure to the substrate passing the applicator head slot at a uniform speed. This is illustrated in FIG. 1. However, under normal mill-operating conditions, there are occasions when the substrate is stopped, which may be for only seconds or may be for minutes or longer. When the substrate stops, the foam delivery must be stopped as well. When this occurs, the pressure differential of the foam within the delivery conduit equalizes, with the pressure at the applicator head being greater than during normal operation and the pressure at the generator end of the conduit being less than the normal delivery pressure from the generator. Thus, when the system is started after such a stop, the higher than normal pressure at the applicator head slot results in a thicker coating delivery due to the higher foam pressure followed by reduction in the coating application as the lower than normal pressure upstream in the conduit further decreases as the foam travels through the conduit to the applicator head slot. As application continues, there is a wave-form fluctuation in the delivery pressure and resulting coating thickness until the system stabilizes in a dynamic condition in which the foam pressure uniformly decreases along the conduit from the generator to the applicator head slot, with a continuous uniform thickness of foam again being applied to the traveling substrate. FIG. 2 illustrates this wave-form fluctuating of coating application along a traveling substrate at startup after the traveling substrate has been stopped. The thickness of the coating C during this initial startup condition can vary from a fraction of the desired coating thickness to a multiple of the desired coating thickness and can extend over a length of, for example, 20 to 30 yards of the substrate. The thickness T of the coating C illustrated in FIG. 2 is exaggerated substantially for clarity of illustration.
This coating application fluctuation at startup is not extreme enough under some coating applications to be of concern, but under some circumstances the variation in coating is unacceptable, resulting in a waste of coating and substrate material that must be discarded before the coated substrate is manufactured into a useable product.
One solution to this problem is to close the applicator head slot and bypass the foam delivery past the applicator head or from the applicator head to discharge into a drain with the system being maintained in its dynamic operating condition during stoppage of the traveling substrate. This, however, creates another problem in the form of waste of the foam material and disposing of the foam under environmental restrictions.
Thus, there is a need for a system that minimizes wasteful coating fluctuations during startup as well as minimizing discharge of unused foam.
By the present invention, an apparatus is provided for applying a chemical foam to a traveling substrate in a manner that minimizes fluctuation in coating thickness and obviates the waste and resulting environmental problems resulting from stops and starts of a traveling substrate as it travels past the foam applicating apparatus.
In the apparatus of the present invention, air and liquid chemicals are mixed in a foam generator to produce foam that is discharged under pressure. A foam applicator head is disposed with a discharge slot extending transversely with respect to a traveling substrate for discharge of foam therefrom to the substrate. A foam delivery conduit communicates with and extends from the foam generator to the foam applicator head to transport foam from the generator to the applicator head with the dynamic pressure of the foam decreasing as the foam travels through the conduit. A plurality of normally open shut-off valves are spaced along the conduit, dividing it into a sequence of conduit sections. A valve controller is operable to close simultaneously the plurality of valves in response to a stoppage of the traveling substrate to, thereby, seal the foam in each conduit section to maintain a static pressure in each section corresponding to the dynamic pressure of the foam in that section during valve-open transport of foam. The controller is operable to open the valves simultaneously in response to resumption of substrate travel. With this arrangement, there is an equalization of pressure in each section, but the pressures in the sequestial sections decrease from the section closest to the generator to the section closest to the applicator head so that when the valves open and supply of foam resumes, there will be an initial pressure drop between the generator and the applicator head corresponding generally to the dynamic pressure drop during operation. Any minor fluctuations will quickly even out so that the apparatus will resume applying the desired uniform thickness of coating over only a short length of substrate.
At the same time that the valves are closed in the conduit in response to stoppage of the traveling substrate, the slot of the applicator head at the substrate is closed to prevent the remaining foam in the applicator head from being discharged onto the stopped substrate. To avoid deterioration of the foam in the applicator head when the system is stopped, which can cause the development of large bubbles in the foam that adversely affect the initial coating application upon restart, a normally closed by-pass outlet is provided in the applicator head operated by a by-pass discharge valve that opens in response to stoppage of the traveling substrate. Functioning with this by-pass discharge valve for the purpose of maintaining dynamic foam flow through the applicator head during stoppage of the substrate and stoppage of foam flow through the foam conduit, a by-pass conduit is provided for transporting foam from the generator to the applicator head bypassing the valves for continuous transport of foam to the applicator head and out the by-pass discharge outlet during stoppage of the substrate with the applicator head slot closed, thereby maintaining desired foam characteristics in the applicator head for resumption of foam application upon startup of the substrate. The by-pass conduit is substantially smaller in cross-section than the foam conduit so that only a minor portion of the normal quantity of foam is transported through the by-pass conduit. Also, the by-pass conduit is open continuously both during normal operation and during stoppage, with the foam flow through the by-pass conduit combining with the flow through the foam delivery conduit during normal operation to provide the foam being applied to the traveling substrate. In the preferred embodiment, the cross-section of the by-pass conduit relative to the cross-section of the foam delivery conduit results in approximately 10% to 15% of the total flow of foam through both conduits being through the by-pass conduit.
Preferably, the applicator head includes a foam distribution chamber for distributing the foam uniformly from the foam conduit to the applicator head slot when the slot is open.
Preferably a by-pass foam return conduit is provided for communicating between the discharge outlet of the applicator head and the foam generator for return of foam to the generator during stoppage of substrate travel. This avoids waste of the chemicals in the foam as well as avoiding environmental problems were the foam discharged to a drain.
Preferably, the foam return conduit is connected to a settling tank in which the foam deteriorates to a liquid form and is transported therefrom through a liquid return conduit to the foam generator for recycling.
In the preferred embodiment, there are two settling tanks with a settling tank input valve means operable to discharge foam from the foam return conduit to a selected one or the other of the settling tanks. A discharge valve means discharges the liquid material from one of the settling tanks while the other one is being filled. The settling tank input valve means and the discharge valve means are controlled by a valve control responsive to the level of fill in the settling tanks that closes flow to a filled settling tank, opens flow to the other settling tank, closes the flow from that other settling tank and opens flow from the filled tank to a liquid return conduit when the foam has settled to a liquid. A feed pump in the liquid return conduit pumps liquid from the settling tanks to the foam generator.
Other features and variations of the present invention will be apparent from the following detailed description and the accompanying drawings.